This invention relates to a method of producing a gas sensor element having a porous film of a gas sensitive material formed by a thick film technique on a ceramic substrate so as to cover electrode films precedingly formed on the substrate, the method including steps for improving the strength and stability of the contact between the gas sensitive thick film and the electrode films.
In gas sensor elements of the aforementioned type, viz. thick film gas sensor elements, usually the gas sensitive thick film is formed on a flat and smooth surface of a ceramic substrate so as to make contact with both the substrate surface and the precedingly formed electrode films. A problem about this construction is that the thick film is liable to partially peel from the substrate surface during practical use of the gas sensor due to a difference between the thermal expansion coefficients of the thick film and the ceramic substrate. If the contact of the gas sensitive thick film with the electrode films is not good the internal resistance of the gas sensor element increases during operation of the sensor element, which causes lowering of the sensor output and adversely influences the accuracy of the control system in which the sensor is used. Therefore, it is important to improve the strength and stability of the contact to thereby minimize an increase in the internal resistance of the sensor element during actual operations.
To enhance the strength of the contact of the gas sensitive thick film, JP-A 60-93949 proposes to intentionally produce minute undulations in the ceramic substrate surface before forming the thick film by applying ceramic grains to the substrate surface. However, even if this measure is taken, the tightness of contact of the thick film with the underlying electrode films is still insufficient in a microscopical sense, so that the internal resistance between the thick film and the electrode films is liable to deviate from a standard value. Besides, there is another cause of an increase in the internal resistance and resultant deterioration of the gas sensing performance during use of the gas sensor element. That is, impurities contained in the substrate and/or the thick film migrate to accumulate in the micropores of the thick film in the region adjacent the interface between the thick film and each of the underlying electrode films.
Concerning the problem of the increase in the aforementioned internal resistance, we recognized that because of the contact of the gas sensitive thick film and the electrode films in a plane there occurs concentration of an interfacial tension in the directions parallel to the plane, and that due to the concentration of the interfacial tension the tightness of contact between the thick film and each electrode film becomes worse as time elapses with a resultant increase in the contact resistance.
JP-A 62-5165 (Jan. 12, 1987) proposes to interpose a conductor between the gas sensitive porous thick film and each of the underlying electrode films by depositing platinum or an alloy or mixture of platinum with another noble metal. The deposition of the noble metal is accomplished by impregnating the porous thick film with a solution of a nobel metal compound and then making a heat treatment in hydrogen gas at about 700.degree. C. By the deposition of the noble metal conductor at the interface between the thick film and each electrode film, the manner of contact between the thick film and each electrode film changes from two-dimensional contact to three-dimensional contact. Therefore, the strength of contact augments and the concentration of tension in the directions parallel to the interface is relieved, and consequently the contact resistance is stabilized. However, it has been revealed that the manner of the deposition of the noble metal is greatly influenced by the conditions of the heat treatment in a reducing gas atmosphere. In this regard we have already proposed in JP-A 63-231255 (Sep. 27, 1988) to carry out the heat treatment of the thick film impregnated with a solution of a platinum compound in hydrogen gas at a temperature in the range from 60.degree. to 180.degree. C. After that we have reached the present invention by finding still better conditions of the heat treatment for establishing very good and stable contact between the thick film and the electrode films.